1. Field of the Invention
The present invention is related to using an enhanced CMOS circuit to form a driver circuit of a DC motor, and more particularly to an enhanced CMOS circuit that is able to boost current gain, replacing the BiCMOS circuit for the part of the driver circuit, in order to produce sufficient output current to drive a DC motor of a portable CD player. Also, the present invention enables the combination of different control circuits to be formed on the same chip using an MOS process with lower production costs.
2. Description of Related Art
A BiCMOS circuit contains both bipolar junction transistors and CMOS transistors, allowing for advantages of both processes to be exploited. Bipolar circuits are inherently faster than the CMOS, and have shorter signal delay and faster switching speed, making them suitable for high frequency applications such as operation amplifiers, but CMOS circuits are preferred where low power dissipation and high packing densities are required such as for microprocessors and memory ICs. However, the production costs of hybrid BiCMOS circuits are higher than those for CMOS circuits. For those applications that do not require high frequency and fast switching speed, the CMOS circuits are more frequently used.
But conventional CMOS transistors have their own limitations. The small output current, due to the inherent driving impedance, is insufficient to drive a DC motor or other digital motors. To solve that problem, a number of alternatives are to be described below.
1. Increasing the channel width of CMOS: generally a CMOS channel width is about 6,000 um. If the channel width is increased to 16,000 um, the driving impedance (Ron) through the conduction channel of CMOS transistors can be lowered to boost current gain across the CMOS transistors, thus the output current can be increased. Using this method will increase the size of the integrated circuit, which is contrary to current efforts to downsize components.
2. Increasing the gate-to-source voltage (VGS) of CMOS: if the gate-to-source voltage (VGS) of CMOS is increased, driving impedance through the conduction channel of CMOS can be lowered to boost current gain, so output current will be increased. However, the CMOS generally exhibits low voltage endurance, thus it poses a challenge for circuit designers to maintain the operating voltage of CMOS circuits below 5V.
The input impedance of CMOS transistors is exceptionally high. Hence, using the conventional CMOS technology, output current cannot be effectively increased to meet the requirements of large current applications. To get around the problem, designers choose to use CMOS for the part of control circuit, and BiCMOS for the part of driver circuit. But then the combination of BiCMOS and CMOS transistors on the same chip requires much more complex processes.